Heavy duty diesel engines typically employ a camshaft actuated valve or injector train to convert the rotary motion of the camshaft into the synchronized reciprocating motion required to operate the cylinder head valves and fuel injectors so that the valves and injectors open and close at optimum intervals. The fuel injection interval, in particular, must be very carefully timed so that the high pressure required to achieve the maximum possible atomization of the injected fuel is produced. This is usually achieved by mounting an injector cam on the camshaft to rotate in a fixed relationship with the crankshaft. A rolling cam follower assembly, which includes a pin-mounted roller, rides on the cam and translates the rotational movement of the camshaft to an injector train pushrod or pushtube and through the injector train to a fuel injector. The valve trains for the valves operate in a similar manner to provide the reciprocating motion required for the operation of these structures. U.S. Pat. Nos. 4,090,197; 4,962,743 and 5,011,079, owned by the assignee of the present invention, illustrate this type of fuel injector and valve train.
During engine operation the roller element of the drive train rolling cam follower is continuously contacted by a rotating cam mounted on the camshaft. As a result, the roller rotates constantly when the engine is in operation. Most cam follower rollers are rotatably mounted in a cam follower assembly on a pin. The roller is typically made of steel, the cam follower assembly supporting lever is cast iron, and the pin securing the roller to the lever is bronze. The stresses produced on the cam follower assembly elements, particularly those at the interface between the cam follower roller and the pin, can cause, among other things, undesirable wear of the pin and adversely affect the rotational stability of the roller. Optimal camshaft cam and roller interface conditions cannot be maintained unless the roller is allowed to rotate freely. Without this freedom of rotation, such interface conditions as the maintenance of a lubricant film, the load distribution and the rolling contact may suffer significantly. The susceptibility of the roller pin to wear thus ultimately reduces cam life and engine efficiency because a valve or injector train with a worn pin cannot effectively support a roller to drive a valve or injector to operate with the timing accuracy required.
In addition to the timing problems that accompany worn roller pins, engines that utilize rolling cam followers experience reduced service life from damage to other valve or injector train components when the engine is shut off and started frequently. This reduction in service life results from damage to the camshaft cams and roller followers, primarily from material transfer, known as galling or scuffing, between the surfaces of the camshaft cams and the oscillating follower roller. If left unchecked, such galling eventually leads to spalling of the cam and the functional failure of the engine. High demand traction forces between the cam lobe and roller produces cam galling or scuffing. In extreme cases "skidding" of the roller may result, although damage to the cam may occur without skidding. This condition is particularly severe on startup and shutdown of the engine due to several factors, most of which are related to insufficient oil at the roller-pin interface. The low rotation speeds, characterized by an absence of hydrodynamic oil film, create high friction between the roller and the roller support pin. The insufficient oil supply at the pin-roller interface produces, at best, only a thin oil film, and wear of the pin material under this condition eventually leads to a more conformal contact with the roller, which further reduces the oil film thickness. Reducing the wear between the pin and roller during engine startup and shutdown would obviate these problems.
The prior art has proposed increasing the life of camshaft-mounted cams by forming the cams of wear-resistant materials. U.S. Pat. No. 5,082,433 to Leithner, for example, discloses forming cams from a sintered alloy with a hardened matrix of interstitial copper, consisting of 0.5 to 16% by weight molybdenum, 1 to 20% by weight of copper, 0.1 to 1.5% by weight of carbon and, optionally, of admixtures of chromium, manganese, silicon and nickel totalling, at most, 5% by weight, the remainder being iron. Cams and other similar internal combustion engine components, e.g., rocker arms, formed from the foregoing alloy are disclosed to be resistant to sliding wear.
U.S. Pat. No. 5,529,641 to Saka et al. discloses improving the scuffing and pitting resistance of cams on a camshaft by forming the cams of a cast iron comprising 3.0 to 3.6% by weight carbon, 1.6 to 2.4% by weight silicon, 0.2 to 1.5% by weight manganese, 0.5 to 1.5% by weight chromium, 1.5 to 3.0% by weight nickel, 0.5 to 1.0% by weight molybdenum, 0.0003 to 0.1% by weight of at least one chilling promoting element selected from the group consisting of bismuth, tellurium and cerium, and the balance iron and unavoidable impurities. Neither the Saka et al. nor the Leithner patents suggests that the material forming the pin mounting a cam-contacting roller in the engine drive train affects cam wear.
U.S. Pat. No. 5,246,509 to Kato et al. discloses a wear-resistant copper base alloy for forming a floating bush bearing in an engine turbocharger. This alloy comprises 1.0 to 3.5 wt % manganese, 0.3 to 1.5 wt % silicon, 11.5 to 25 wt % zinc, 5 to 18 wt % lead, and the balance substantially copper and incidental impurities. Although this alloy is stated to withstand the operation at high sliding speed and high temperature in a highly-corrosive condition typically encountered in a turbocharger, it is not suggested that this alloy would resist the rolling wear or the conditions encountered by a pin supporting a cam-contacting roller.
U.S. Pat. No. 4,462,957 to Fukui et al. discloses roller and pin structures, wherein both structures are made of wear-resistant alloys. The pin and roller structures described in this patent are used in the guide mechanism of a nuclear reactor control rod. Not only are the rollers fixed so they do not contact a cam-like structure, but there is no suggestion that the material from which the pin is formed affects the life of any structure that does not contact the pin.
Two "Alloy Digest" publications describe copper alloys useful as bearings, bushings and the like. Mueller Alloy 6730 is composed of 60.5% copper, 2.5% manganese, 1.0% lead, 1.0% silicon, and 35.0% zinc. Copper Alloy No. C67300 is composed of 58.0 to 63.0% copper, 2.0 to 3.5% manganese, 0.5 to 1.5% silicon, 0.40 to 3.0% lead, 0.50% max iron, 0.30% max tin, 0.25% max nickel, 0.25% max aluminum, and the remainder zinc. These alloys are stated to be useful for forming bearings, bushings, cams and idler pins. It is not suggested that either of these alloys could be used to form a pin for a cam-contacting roller to enhance the durability of a cam which is not directly contacted by the roller. It is also not suggested that either of these alloys combine high wear and corrosion resistance in the presence of lubricant additives.
In one commonly available cam follower roller and pin assembly the roller is made from steel, and the pin is made from a leaded phosphor bronze. It has been discovered, however, that this pin material is not sufficiently low friction or wear-resistant or corrosion-resistant in the presence of lubricant additives to prevent cam galling or failure.
The prior art, therefore, has failed to provide a pin that is low friction, wear-resistant and corrosion-resistant in the presence of engine oil additives for rotatably mounting a cam follower roller in a drive train of a heavy duty internal combustion engine made of a material which is corrosion-resistant, is capable of embedding hard debris without scuffing, and is capable of carrying the mechanical loads imposed on the cam follower. The prior art has further failed to provide a cam follower roller pin made from a material that prevents cam failure and enhances cam durability.